The R-T-B based magnet (R is a rare earth element, T is Fe or Fe with part of it replaced by Co, and B is boron) comprising a tetragonal compound R2T14B as the major phase is known to have excellent magnetic properties, and has been considered as a representative permanent magnet with good performances since it was invented in 1982 (See Japanese Laid-Open Patent Publication No. Sho 59-46008).
In particular, the R-T-B based magnets in which the rare earth element R consists of Nd, Pr, Dy, Ho or Tb have large magnetic anisotropy fields Ha, and have been widely used as permanent magnet materials. Of those, the Nd—Fe—B based magnet having Nd as the rare earth element R is widely used in people's livelihood, industries, transportation equipment and the like, because it has a good balance among saturation magnetization Is, Curie temperature Tc and magnetic anisotropy field Ha. However, in recent years, since the uses of the R—Fe—B based magnet are expanding and the consumption amount of Nd or Pr or the like is dramatically increasing, it is strongly required that the efficient utilization of Nd or Pr or the like as the valuable resources is achieved and the material costs of the R—Fe—B based magnet are controlled low.
On the other hand, as the R-T-B based magnet comprises a rare earth element which is easily oxidized and an iron as the main component, its corrosion resistance is relatively low, thus its magnetic properties would deteriorate due to oxidization. Therefore, the forming of various protection films on the surface of the magnet body is performed. If the adhesion strength between the magnet body and the protection film is low, the protection film would peel off according to the conditions during using, causing corrosion of the magnet body. Therefore, the adhesion strength between the magnet body and the protection film is required to improve. The adhesion strength between the magnet body and the protection film is due to both physical and chemical binding force, but it depends on the chemical properties of the magnet body and the protection film essentially. That is, adhesion property is influenced by the composition of magnets.